SIGMA OPH (Sigma Ophiuchi). Among the most common kinds of naked eye stars are class K orange-colored giants that because of their intrinsic brightnesses stand out far beyond their actual numbers as compared to fainter dwarfs like the Sun. They are everywhere, from Arcturus and Aldebaran through Unukalhai (Alpha Serpentis). Take them from the sky along with the also-common class A dwarfs such as Vega and Sirius, and most of the constellations would be devastated if not eliminated. But even among the K giants, a few stand out. Fourth magnitude (4.34) Sigma Ophiuchi, in north central Ophiuchus four degrees to the west of Cebalrai (Beta Oph), is one of them. As a K2 "bright giant," and even classified by some as a modest K3 supergiant, it's unusually luminous. Just how luminous, however, is open to some question. At a hefty distance of 900 light years (give or take 70) and not that far off the Milky Way, the amount of dimming by interstellar dust is rather poorly known. From a middle ground of 0.4 magnitudes worth of dimming, the star's distance, and a well determined temperature of 4085 Kelvin (needed to account for a fair bit of infrared radiation), we find a luminosity of 4000 times that of the Sun. Temperature and luminosity then combine to yield a radius 127 times that of the Sun, or 0.59 Astronomical Units, 82 percent the size of Venus's orbit. Sigma Oph appears to be something of a "hybrid star," one with two kinds of wind, one fast and magnetically driven (like the Sun's), the other slow and driven by radiation. Adding to Sigma's clouded character, it is not at all clear just where the star stands in the context of stellar evolution. Its parameters are consistent with it having a dead helium core that is about to fire up to fuse to carbon, it could be quietly fusing its helium already, or it could be in the early advanced stages of gianthood, brightening with a dead carbon core and preparing to become something like Mira in Cetus (though there seems to be no kind of brightness variation). Given that uncertainty we can pin the mass down only to between 6 and 8 times solar. With more interstellar dimming, the mass could be even greater. Starting life as a hot class B dwarf only a few tens of millions of years ago, the star will probably die as a massive white dwarf in the mold of Sirius B or beyond (perhaps even one made of neon and oxygen rather than carbon and oxygen), and all alone, with no known companions.

Written by Jim Kaler 7/12/13. Return to STARS.